title: Cellular metabolic regulators: novel indicators of low-grade inflammation in humans.
abstract: OBJECTIVE
  The Toll-like receptor 4 (TLR4) ligand endotoxin triggers robust systemic inflammatory responses in humans at doses equal to or greater than 1 ng/kg. In this study, we tested the hypothesis that evidence of TLR4-induced responses would be detectable in leukocytes challenged with endotoxin doses that are below the threshold needed to trigger a characteristic systemic inflammatory phenotype in humans.
  
  
  METHODS
  Subjects were challenged with endotoxin at 1, 0.5, or 0.1 ng/kg (n = 5 per dose). Systemic responses were monitored for 24 hours. Blood samples, collected at designated intervals, were used to determine plasma cytokines levels, total and differential leukocyte counts, expression of leukocyte cell surface receptors, and changes in the leukocyte transcriptome. Western blotting was used to determine changes in leukocyte protein expression.
  
  
  RESULTS
  We found that in vivo endotoxin at doses below 1.0 ng/kg triggers weak and variable responses in humans. In marked contrast, we show that endotoxin at a concentration as low as 0.1 ng/kg triggers a transient decline in cellular ATP levels in leukocytes. This is associated with the appearance of a unique protein expression signature in leukocytes. The protein expression signature includes 3 prominent features: (i) AMP-activated protein kinase subunit α (AMPKα) degradation, (ii) increased hypoxia inducible factor-1 (HIF-1) α expression, and (iii) autophagy, collectively indicative of a regulated metabolic response. An indistinguishable response phenotype was observed in human leukocytes treated with endotoxin in vitro.
  
  
  CONCLUSIONS
  These data demonstrate for the first time in humans that a TLR4 ligand concentration that is below the threshold needed to trigger clinically evident systemic inflammatory manifestations initiates a transient decline in ATP levels, AMPKα degradation, HIF-1α expression, and autophagy in leukocytes. This establishes that low-grade TLR4 activation exerts control over leukocyte metabolism in the absence of systemic inflammatory indicators.
authors.affiliation: From the Department of Surgery, University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School, New Brunswick, NJ.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Haimovich
authors.forename: Zhiyong
authors.initials: Z
authors.lastname: Zhang
authors.forename: Jacqueline E
authors.initials: JE
authors.lastname: Calvano
authors.forename: Steve E
authors.initials: SE
authors.lastname: Calvano
authors.forename: Ashwini
authors.initials: A
authors.lastname: Kumar
authors.forename: Marie A
authors.initials: MA
authors.lastname: Macor
authors.forename: Siobhan
authors.initials: S
authors.lastname: Corbett
authors.forename: Susette M
authors.initials: SM
authors.lastname: Coyle
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Lowry
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D015153:Blotting, Western; D016207:Cytokines / Q000097:blood; D004731:Endotoxins / Q000009:adverse effects; D005786:Gene Expression Regulation; D006801:Humans; D007111:Immunity, Cellular / Q000235:genetics*; D007249:Inflammation / Q000097:blood / Q000235:genetics* / Q000276:immunology; D007958:Leukocyte Count; D007962:Leukocytes / Q000187:drug effects / Q000276:immunology / Q000378:metabolism*; D012313:RNA / Q000235:genetics*; D051197:Toll-Like Receptor 4 / Q000096:biosynthesis / Q000235:genetics*
chemical_list: D016207:Cytokines; D004731:Endotoxins; D051197:Toll-Like Receptor 4; D012313:RNA; D000255:Adenosine Triphosphate